[CentOS] Port Forwarding

I'm having trouble getting port forwarding working on my one box.

I have 2 incoming internet connections.
I have 2 servers on these connections.

pubinternet                                           privinternet
eth0:x.x.x.x                                           eth0:y.y.y.y
eth1:10.10.10.1                                     eth1:10.10.10.4

I need to be able to port forward traffic from y.y.y.y to the box on
10.10.10.1. I have my firewall configured (I use gshield) but
10.10.10.1 does not seem to answer when I try and test.

I use gshield all the time for other servers with only 1 connection,
and it works awesome. I can't seem to get it to forward traffic from
y.y.y.y to 10.10.10.1.

10.10.10.1 and 10.10.10.4 are connected together with a switch, and I
am able to telnet to 10.10.10.1 from 10.10.10.4 on port 25.

Is there some step with networking that I am missing in getting this to work?

Thanks.
-- 
-=/>Thom